A typical home entertainment audio system includes two or more loudspeakers that serve as transducers to convert electrical signals into acoustic energy to be heard and enjoyed by the listener. A significant advance in speakers has been the use of planar diaphragm or ribbon transducers. A planar diaphragm transducer includes a pair of spaced apart permanent magnet arrays with a movable diaphragm or ribbon therebetween. Electrical conductors are supported on the movable diaphragm and receive the driving signal. Accordingly, the diaphragm moves inwardly and outwardly in a push-pull arrangement with respect to the magnet arrays to convert the electrical energy into acoustic energy or sound. Such planar transducers are disclosed in U.S. Pat. Nos. 5,901,235 and 6,760,462, for example.
Continuing improvements in such planar diaphragm transducers have been forthcoming particularly with the use of improved permanent magnet materials, such as neodymium. A planar transducer is excellent sonically, has a fast response due to a low moving mass, has low distortion, has good sensitivity, has high power handling capability, and remains a fairly constant resistive load thereby not needing a matching transformer. Such planar diaphragm transducers are used by several loudspeaker manufacturers including VMPS Audio Products of El Sobrante, Calif., the assignee of the present invention. Typically, one or more cone-shaped drivers or conical diaphragm transducers may be included within a common housing or baffle with the one or more planar diaphragm transducers.
Another feature relating to loudspeaker performance is directivity. In particular, horizontal directivity is a measure of amplitude linearity for different frequencies over a horizontal angle in front of the loudspeaker. A stereo system, for example, desirably produces a virtual image for the listener by taking advantage of the localization ability of human hearing. Accordingly, relatively constant horizontal directivity is desired over a fairly wide angle from the axis of the loudspeaker. This may also accommodate multiple listeners.
Several attempts have been made in the past to address and improve directivity. For example, U.S. Pat. No. 4,134,471 to Queen discloses a loudspeaker including a radial horn that radiates a spherical sector over 360 degrees through a horizontal plane. One or two speakers are mounted so that they produce a pulsating cylindrical wave to feed into the radiator and an inverted conical member is mounted in the transition portion between the pulsating cylinder and the output horn. This output is blended with similar wavefronts produced by a low frequency loudspeaker that is acoustically associated with a vented housing.
U.S. Pat. No. 6,513,622 to Gelow et al. is directed to a cinema loudspeaker system and includes, for example, a midrange frequency module that is an integrated multi-band waveguide assembly configured to provide a vertical array of four contiguous specially-shaped waveguide regions each driven by a cone type transducer driver. The required defined coverage is accomplished through a combination of special shaping of the waveguide directing surfaces with vertical asymmetry to provide controlled directivity vertically and horizontally, and frequency-selective filtering in a passive network that accomplishes the required overall coverage by splitting the drive power into two paths with different special transfer functions allocated to the lower two transducers as a low-frequency portion and the to the upper two transducers as a high-frequency portion of the midrange assembly. The four drivers are separated by partitions shaped with strategic spacing dimensions, each driver working into an individual waveguide throat portion, and each directed at an inclined angle downwardly from horizontal, to optimize defined coverage uniformity. The throat portions combine smoothly into a common flared mouth portion that extends to the substantially rectangular shape of the front outline of the midrange module.
Despite continuing advances in loudspeakers, and particularly in the use and improvement of planar or ribbon diaphragm transducers, such may not have relatively constant directivity over larger angles.